Spectrophotometer attachment for rejecting specular reflection



1 1944- w; A. SHURCLIFF 2,364,825 SPECTROPHOTOMETER ATTACHMENT FOR REJECTING SPECULAR REFLECTIONS Filed Feb. 13, 1942 INVENTOR. I 2 W/(l/AM ,4. J/ll/ICl/f/j ATTORNEY.

Patented Dec. 12, I944 UNITED. STATES, "PATENTS osncs T SPECTROPHOTOMETER ATTACHMENT FOB REJECTING SPECULAR REFLECTION William A. Shurcliff, Plainfleld, N. 1., assignor to American Cyanamid Company, New York, N. Y.,

a corporation of Maine Application February 1:, 1942 Serial No. 430.713

(c1. ca -14) 5 Claims.

This invention relates tospectrophotometric measurement of reflectance particularly to the measurement of the body reflectance of specularly reflecting samples.

In the past considerable information with regard to the composition of various samples has been obtainable through the use of flickeringbeam type recording. spectrophotometers. Examples of such spectrophotometers are well illustrated in U. S. Letters Patent No. 2,107,836 and 2,126,410 issuedbn February 8', 1938, and August 9, 1938, respectively, to Orrin W. Pineo. Even the best of such spectrophotometers, however, will not give immediately useful readings with sampleshaving a high gloss such as hard finished papers, certain inks, lacquers, .paints, molded the specularly reflected light by placing a lens or resins, ceramics, and other materials exhibiting similar phenomena. It is usually desirable to determinethe body reflectance dependent upon the structure or composition of the material itself rather than the total reflectance which is obtained when a sample is run in-a conventional type apparatus. In the case of samples where there is an appreciable surface gloss, specularly reflected light may comprise a large or even a major proportion of the total reflectance of the sample at a given wave-length.

As pointed out the specularly reflected light may comprise a major portion of the total reflectance. In addition it should be broughtout that in most circumstances there is a certain minimum below which the total reflectance will never fall however low the body reflectance. For

larly reflected light, is rejected from the sphere by returning it out through the entrance window.

The second of these methods forms thesubject matter of my co-pending application, Serial No.

430,720 flied of even date. now United Sta Letters Patent No. 2,347,067.

In the past it has been proposed to eliminate :0 the noreflector ordinarily must have a curved surface to focus the light on the window and the sample therefor must be one adapted to be fitted over this curve. This limited the types ofsamples which could be so tested to those which are on a flexible surface which can bebent around a curve or to materials such as inks, paints. and pigments and the like'which could be directly placed on the back of the zero ower lens. t

The present invention in general comprises a method and means whereby the body reflectance of samples having appreciable specular reflectonce may be evaluated whether the materials example, with plastics .such as Bakelite this minimum usually occurs somewhere about 4%. Therefore, when only the body reflectance is desired, the readingsobtained from a-conventional type apparatus with the sample held in the conventional manner may be in error by 4%-or more even with a very light sample. With dark samples the error will often beappreciably more than It is therefore readily apparent that for many purposes the'elimination of this surface reflectance becomes highly important.

There are two principalmethods of eliminating the specularly reflected light from the reading obtained with the spectrophotometer. The first:

of these is to reject specularly reflected light comingfrom the surface of the sample from the interior of the integrating sphere. The second of so and 1 through whichj'enter the divergent these comprises the provision :of'some means within the integrating sphere for eliminatingtha effect of, the specular-reflectance.

meet the requirements imposed by the prior art or not. The invention will be more fully discussed in connection with the drawing in which: Figure 1 is a partially diagrammatic section of a conventional integrating sphere-with the reference'sample and sample to be tested in place: Figure 2 shows an arrangement using aspecially prepared sample;

Figure 3 shows one means elf-rejecting specular reflection;

riguresi and 5 show various forms of the pre- 1' ferred embodiments of the presentinyentibn:

um 6 of Figure 4; showing the oil layer in detail.;-

represents a greatly enlarged portion 7 Referring to Figure 1, I represents the integrate.

ing sphere of scon'ventionalflicker type spectror.

photometer having the, two entrance windows! of light represented by'thelines l and I."

sourceof these'beams forms no-partlof the ,present invention, is conventional andisl not shown. I Thebeamyl falls"uponastsndard reflectance 'ss sample 0. of magnesium carbonate or the like, 7

mounted in sample window 2' and is diffusely reflected into the interior of, the sphere being picked up by a photo tube or photo cell I in the conventional manner. The beam 5 is represented as falling normally upon a sample 8, mounted in sample window 3'.. Both the standard sample 8 andthe sample to be tested 8 are held in place by some mounting means such as spring clips I I and 'Interposed between the adapter and the sample is II or the like, which being conventional are not shown in the remaining figures. A part of the light will be specularly reflected from the plane surface of the sample as represented by the lines 9 and I0, fall upon the surface of the integrating sphere and be picked up by the light receiving device. The remainder of the-beam 5 will be diffusely reflected in the same manner as is the light from the reference sample. Since it is one of the purposes ofthe present invention to provide a means whereby only the diffusely reflected light is to be compared with the total diffused re.-

flectance from the reference sample, the diagram will serve'to emphasize the type of objectionable reflectance which it is desired to eliminate. As will be seen from Figure 1 this is not done in a conventional type machine with conventional entrance window 3 whereby it will pass from the 1 integrating sphere and will not-cause an inaccua thin layer of a suitable 011 I3, having no appreciable spectral absorption and an index of re-' fraction-between that of the sample itself and th material or which the adapter is made. The

' adapter device may be held in place by any suitable means as by making the adapter integral with the reflectance sample holder. In most cases-however, no special holding device will be requiredsince the adhesivequality of the oil is such that the' adapter will be held on the surface of the sample without any special support.

The use of a thin adapter of this type may be subject to a certain amount of objection on the grounds that the concave surface of the adapter will increase the internal reflection of the sample rate measurement. In this example, rather than f form the sample around an adapter or noflector, the "noflector has been cut in the sample.

While this modification" can be made to give fairly accurate results it has several serious drawbacks. First, it requires that the sample be ca pable of having its surface deformed in the necessary curve. Many substances are not so adaptable and many case considerable time is reguired for the cutting. Secondly, the curve must be very accurately formed and the sample very accurately placed, otherwise. it will focus the specularly reflected light upon a reflecting portion of the interior surface of thesphere and cause an abnormally high total reflectance reading since under normal conditions a certain or the otheroi' the, entrance windows 2 or '3.

50. of the sample would have passed out through one Figure 3 involves a modification of the in- "tegrating sphere of a conventional spectrophotometer to enable rejection of the specularly reflected light. In this case it is done by enlarging the entrance window through which the sample .assembly and cause thepercent of reflectance recorded by the apparatus to be too low. However, such little decrease as .will occur in recorded reflectance may be substantially eliminated by the use of slightly modified adapters. An example ofsuch modified adapters is shown at II in Figure 5. The critical feature in such'modifications is that the rejector surface of the adapter is farther away from the surfaceof the sample itself and subtends a smaller angle at the-sample ple, the adapter may, like adapter I! of Figure-5,

be merely a thickened form ofthe thin adapter I I2 of Figure 4, and be tapered slightly to subtend a smaller angle. The thickness may be varied as desired. However, it is desirable to keep the amount of diflusely reflected light which escapesfrom the sphere along the adapter at a minimum.

This'is best accomplished as" noted by tapering the adapter. The external shape of theadapter may be varied'at will so that the taper may be a smooth curve as in Figure 5,-may be a straight conical section, or may be carried out in a series of steps as may-be thought best or be most convenient to attach to theinstrument. When using adapters, such as adapter II, it

, may be necessary to compensate for'the amount of light reflected from the front. face of the for the standard beam may be used, perhaps the beam enters the sphere to such an extent that O and llwill pass out through the window. While .this modification may be used to produce fairly I results it has the drawback that the structure of the sphere mustbe permanently'althe speeularly reflectedlight represented 'by lines this enlargement ofone or both of theentrance I 7 windows decreases the effective areas inside the sphere.

. rigure (shows one modification of a means capable of accomplishing the object of the present invention without modifying. thesphere. A

most convenient being a wedge diaphragm or a neutral filter. By running'a'blank. on thespparatus using two similar samples, the machine may be readily adjusted so that equal amounts of;light'will fall'on'both samples.

. I Particularlyimportant in connection withthe pr esent invention is theuseof the oil layer It.-

Its-advantage iselearly shown in Figure 8. The oil 'layer effectively seals the-Joint between adaptenand sample and therebyeliminates subreflectancefrom the face. of the samplewhether Thus,

a sample which hasbeen accidentally sc'ratcl'iedv need not bereiected and samples which have matte, rough-ground or surfaces as shown by the surface is in Figure ti-maybe used without. having surfaces polished orotherwise suitable t adapter 12 having a "modified aswould have been n'incarrying out any of the procedures proposed prior to the present invention.

for the specular reflectance or when it is de- The adapter itself may be made of any suitable transparent material such as glass or a polymerized acrylic resin. The presence of the oil thereon overcomes the necessity for having the surface of the adapter in contact with the oil an exact plane. The oil will overcome any slight irregularities in the adapter surface itself and will even permit the use, if so desired, of an adapter having both principal surfaces in the form of a curve- The latter type, however, is not as desirable as is an approximately flat surface since it is more difllcult to place in the proper position. The diameter of the adapter is not critical. It must be sufliciently great so that the 'whole beam will fall on the surface of theadapter but otherwise may be of almost any desired size. If the adapter is larger than the sample window, it may be mounted on the exterior of the sphere. A preferable diameter is one which though greater than the beam diameter is smaller than the window since this position.

I claim:

1. In a flickering-beam type spectrophotometer,

is the easiest size to properly an integrating sphere having at least one opening for the admission of light into the sphere; at'least'one sample mounting window; a transparent adapter located in said window, said adapter having one face forming a substantially continuous surface with. that portion of the ex-' ternal sphere surface surrounding and adjacent to said window, the opposite face of the adapter being shaped to focus the specular component of the total reflected light resulting from incidence of substantially paraxial light thereon on said light-admitting opening and a mounting means adapted to hold a sample in contact with the :urface formed by said adapter and sphere surace.

2. In a method of evaluating the body reflec-,

tance of a sample in which reflected light resulting from incidence of a beam of substantially paraxial light on the sample is concentrated on a light-responsive element by means of a light-- integrating chamber, a portion of the surface of which is substantially non-reflecting {an improved method of preventing the specular component of the total reflected light from being-reflected onto said light-responsive element by the light-integrating chamber which comprises the steps of covering thesurface of the sample with a thin layer of a non-selectively absorbing oil having an index of refraction approximating but not exceeding that of' the sample, placing the oil-covered surface of the sample in optical contact with one surface of a transparent, rigid adapter, the opposite surface of said adapter being shaped to focus the specular component of the total reflected light on said non-reflecting portion of the interior surface of the chamber when placed in said chamber in the path of the substantially paraxial light beam, said adapter having an index of refraction approximating but not exceeding that'of said oil layer, and placing the adapter and sample in said chamber in the path of said substantially paraxial light beam.

3. A method according to claim 2 in which the specular component of the reflected light is focused on an opening in the surface of the lightintegrating chamber.

4. A method according to claim 2 in which the specular component of the reflected light is.

at least one sample mounting window; a transparent adapter located in said .window, said adapter having one face forming a substantially continuous surface with that portion of the external sphere surface surrounding and adjacent to said window, the opposite face being shaped to focus the specular component of the total reflected light resulting from incidence of substantially paraxial light thereon on a small area of a nonreflecting portion of the-interior surface of the sphere, and a mounting means adapted to hold a sample in contact with the surface formed by said adapter and sphere surface.

WILLIAM A. SHURCLIFI'. 

